This invention relates generally to flame-resistant thermoplastic materials. More particularly, the invention relates to flame-resistant thermoplastic compositions comprising a poly(biphenyl ether sulfone), a fluorocarbon polymer, and titanium dioxide, and to a method for flame retarding compositions comprising a poly(biphenyl ether sulfone). The composition is useful for a wide variety of applications, such as to make aircraft interior parts.
The quest for flame retardants effective for use with aromatic thermoplastics such as polyarylates and polysulfones has continued for decades. Since the early invention of resins such as polycarbonates and polysulfones more than two decades ago, as well as of polyethylene terephthalates much earlier, the art has sought improvement in the methods and compositions used to impart flame retardance to such thermoplastics and to blends comprising one or more such resins. The flame retarding compositions evolved over the years employed halogen-containing compounds, often in combination with a synergist such as antimony trioxide. With the evolution of flammability standards to respond to the increasing demands of commerce for non-burning, low-toxicity resins, there came new challenges, and compositions rated by earlier standards as self-extinguishing were found no longer acceptable because of other deficiencies such as dripping under flame test conditions. In addition, the demand for smaller parts and reduced part thickness increased the exposed surface area and heightened the susceptibility of such articles to contact with flame.
Engineering thermoplastics are used extensively in many components of aircraft interiors, such as wall panels, overhead storage lockers, serving trays, seat backs, cabin partitions, and ducts. Rigid standards for the flame resistance of construction materials used for aircraft interiors are promulgated by government regulation at both the national and the international levels, and the standards are continually being reviewed and tightened. For the United States, these Government flammability 5 standards are set out in the 1986 amendments to Part 25-Airworthiness Standards-Transport Category Airplanes of Title 4, Code of Federal Regulations (see 51 Federal Register 26206, Jul. 21, 1986 and 51 Federal Register 28322, Aug. 7, 1986). The flammability standards, based on heat colorimetry tests developed at Ohio State University, are described in the above-cited amendments to 14 CFR Part 25, incorporated herein by reference. These tests measure the two minute total heat release (in kilowatts per minute per square meter of surface area, KW-min/m.sup.2) for the first two minutes after test start-up as well as the peak or maximum heat release rate (in kilowatts per square meter of surface area, KW/m.sup.2) over the first five minutes after test start-up, when burned under a specified set of conditions. Where the 1986 standards required engineering thermoplastics to have both of these heat release measurements under 100, the new 1990 compliance standards set a maximum of 65 for each of the two heat release measurements. Many of the prior engineering thermoplastics conventionally used in aircraft interior components are not capable of meeting these more demanding 1990 flammability standards. Hence, a need exists to develop new thermoplastic compositions that will be able to meet these very stringent flammability standards without loss of such other desirable features as toughness, chemical, solvent and cleaner resistance, and ease of fabrication into finished components.
Flame retarding additives such as triphenyl phosphate or aluminum trihydrate have been used in combination with engineering thermoplastics to reduce flamrnmability. However, these low flammability additives are frequently found to be ineffective when used with high performance engineering thermoplastics. The low flammability additives may not be compatible with the engineering thermoplastic at additive concentrations needed to achieve significant flame retardance, thereby providing lower flame resistance. The additive also may not be stable at the temperatures needed for processing the particular engineering thermoplastic selected. Furthermore, low flammability additives found useful in one thermoplastic, even though compatible with another engineering thermoplastic, may not effectively lower the flammability of that thermoplastic at practical levels, and the amount of the low-flammability additive necessary to achieve a desired reduction in flammability may adversely affect the physical properties or processibility of the engineering thermoplastic.
Thermoplastic blends consisting of a poly(aryl ether sulfone) and a poly(aryl ether ketone), optionally including a filler and/or a reinforcing fiber, are known; see, for example, U.S. Pat. No. 4,804,697 and EP 297,363. The addition of the poly(aryl ether ketone) to the poly(aryl ether sulfone) was reported to improve the blend properties, particularly their chemical resistance. A study of the phase behavior of such blends was reported in Angew. Makromol. Chem. 171, 119-130 (1989). Mixtures of poly(biphenyl ether sulfones) with poly(aryl ether ketones) are disclosed in EP 254,455 and in U.S. Pat. Nos. 4,713,426 and 4,804,724. These disclosures do not address improvement of the flammability of blends of poly(biphenyl ether sulfones) with poly(aryl ether ketones).
Materials consisting of a fluorocarbon polymer with either a poly(aryl ether sulfone) or a poly(aryl ether ketone) were found to be useful as high performance coatings; see, for example, U.S. Pat. Nos. 3,992,347, 4,131,711, 4,169,227 and 4,578,427. These disclosures are not directed to flame retardant blends of a poly(aryl ether ketone) and a poly(biphenyl ether sulfone). Mixtures of polyarylene polyethers with 0.1 to 30 weight percent vinylidene fluoride-hexafluoro-propene copolymer are disclosed in U.S. Pat. No., 3,400,065. Although several types of poly(aryl ether sulfone) are disclosed as examples of the polyarylene polyethers component, the patent does not describe flame retardant blends of a poly(aryl ether ketone) and poly(biphenyl ether sulfone). Mixtures containing a fluorocarbon polymer, e.g., polytetrafluoroethylene (PTFE), perfluorinated poly-(ethylene-propylene) copolymer, or poly(vinylidene fluoride), with a number of engineering polymers including poly(aryl ether sulfones), are disclosed in EP 106,764. Blends of poly(aryl ether ketones) with non-crystalline copolymers of tetrafluoroethylene are disclosed in U.S. Pat. No. 4,777,214. Composite materials consisting of a mixture of poly(aryl ether sulfone), a fluorocarbon polymer, and carbon fibers or of a mixture of poly(aryl ether ketone), a fluorocarbon polymer, and potassium titanate fibers are disclosed as useful for moldings in Japanese Patents 88/065,227B and 89/029,379B.
EP 307,670 describes mixtures of 10 weight percent of a perfluorocarbon polymer with each of a polysulfone, a polyethersulfone, and a polyether ketone. Improved heat release characteristics were obtained with these mixtures. Also described is the use of the perfluorocarbon polymer, finely divided titanium dioxide or mixtures of perfluorocarbons and titanium dioxide to improve the flammability characteristics of blends of a polyetherimide with a polyetherimide-siloxane block copolymer. The beneficial effect of the titanium dioxide on flame retardancy of these polyetherimide blends is ascribed to interaction between the TiO.sub.2 and the siloxane moiety of the block copolymer portion of the blend. The reference does not disclose flame retardant blends of a poly(biphenyl ether sulfone) with a poly(aryl ether ketone) or a poly(aryl ether sulfone).
Unfortunately, many flame retardant formulations found adequate in prior years for use in demanding applications such as aircraft interiors are no longer acceptable. The use of higher levels of known flame retardants and compositions to improve flame retardant behavior of thermoplastics often effects a detrimental change in processability or ability to withstand exposure to severe environments including chemicals and solvents. There is clearly a need for thermoplastic materials and formulations with improved flame retardance capable of meeting the 1990 U.S. government flammability standards for aircraft interiors that are readily processable both by injection molding and sheet extrusion. Of particularly importance are compositions with the required flammability characteristics having excellent chemical and solvent resistance.